The invention relates to a scanning device for optically scanning symbol codes, comprising a laser source for producing a scanning beam, a first lens for focussing the scanning beam, a detector for detecting light backscattered by the scanned symbol codes, a second lens for focussing the backscattered light onto the detector, first and second rotatable deflection means for both transmitting and deflecting the scanning beam so as to generate a scanning pattern, and drive means for rotating the deflection means.
Such a device, as is disclosed by European Patent 0 032 794 B1, makes it possible to scan symbol codes, such as bar codes, optically and to read out and recognise the information contained in the code by detecting the reflected light. This information relates in general to an item to which the code is affixed. In order to ensure that the code can be reliably read out regardless of the position of the item with respect to the scanning device and the relative position of the code on the item, the item is scanned with a predetermined scanning pattern. That is to say, the light beam describes a pattern on the item, the pattern being chosen in such a way that the scanning of the symbol code in the correct manner is ensured.
In order to generate such a scanning pattern, the known device is provided with deflection means which consist of a hologram and a further deflection element, such as a Fresnel lens. Such deflection means have the disadvantage that, in practice, they have a poor efficiency of, for example, 50%.
In the publication "Thermal imaging systems" by J. M. Lloyd, Plenum Press, New York, 1979, pages 316-320, the general principles of the use of rotating wedges for generating scanning patterns is disclosed. This publication does, however, not suggest the application of these general principles to scanning devices such as bar code scanners, nor does this publication disclose the technical measures necessary to realize a bar code scanner embodying these principles. That is, the said publication does not disclose a compact arrangement of a bar code scanning device. More specifically, the said publication does not suggest a bar code scanner in which both rotatable prisms are mounted in a compact and advantageous structure of pinions and crown wheels, this structure providing the required rotation of the prisms.
The article "High-speed rotating optical scanner" by D. H. McMurty, IBM Technical Disclosure Bulletin, volume 17, no. 2, New York 1974, discloses a bar code reader having rotating prisms for rotating the image of symbols to be read. The prisms are not used for generating a scanning pattern by deflecting a laser beam. Furthermore, the prisms used in this known arrangement are image-inverting Dove prisms. The size and consequently the weight of three such prisms makes this known arrangement less suitable for use in a hand-held scanner. Furthermore, the rather bulky drive means shown in said publication preclude the use as a hand-held scanner.